The invention relates to an adaptive transversal filter having an input for receiving a time-discrete input signal which is representative of three signal levels, first and second delay means for producing delayed versions of each time-discrete input signal, first and second coefficient registers, the first and second coefficient registers for the non-delayed and each delayed version of the input signal containing a specific coefficient of the respective first and second delay means; means for multiplying always one of the coefficients by the associated version of the input signals and means for adding together all the multiplying results obtained to provide a cancellation signal.
Such an adaptive transversal filter, used as a component part of an echo canceller, is described in O. Agazzi et al.: "Non linear echo cancellation of data signals"; IEEE Transactions on Communications, Vol. COM-30, No. 11, November 1982, pages 2421-2433.
In digital transmission via a two-wire link, adaptive transversal filters are used to cancel to the best possible extent in the received signal the echo signal of its own transmitter. Often this echo signal is significantly stronger than the received signal and without adequate echo cancellation an errorless detection of the received signal is not possible. The transversal filter constructs the echo cancellation signal from a linear combination of the most recently transmitted symbol and the N symbols transmitted previous thereto, N being independent of the time during which an own, transmitted symbol still affects the received signal. For each of the N+1 symbols the transversal filter contains its own coefficient in a coefficient register and the contribution of a signal to the echo signal is calculated by multiplying for each symbol the associated coefficient by the symbol level, for example +1, 0 or -1 in the case of a three-level signal.
Transversal filters are not only used for forming an echo cancellation signal, but they are alternatively suitable for use for other cancelling purposes, for example in adaptive decision feedback equalization. For a description thereof reference is made to K. J. Wouda: "An implementation of a 144 kbit/s transmission system for two wire loops"; Trends in telecommunications Vol 1, No 1, pages 55-66.
In order to make it possible to continuously optimize the coefficients in the transversal filter, a transversal filter can be coupled in a manner known per se to an adaptive control loop. For a description of the adaptive control of the coefficients in a transversal filter reference can be made to N. A. M. Verhoecks et al: "Digital echo cancellation for base band data transmission"; IEEE Transactions ASSP, Vol ASSP-27, No. 6, December 1979, pages 768-781.
For the adaptive transversal filter described in said article by Agazzi et al, efforts are made to offer a solution for the problem that in a transmitter when signals of three or more levels are used, the position of the levels relative to the zero level can only be made equidistantly with a limited degree of accuracy. If, for example, for three-level signals the +1 level has, relative to the 0-level, an amplitude which deviates from that of the -1 level relative to the 0-level, then, in a conventional adaptive transversal filter a product will be produced on multiplication by a predetermined coefficient for a previously transmitted symbol which for a -1 symbol differs from that for a +1 symbol, which results in unwanted residual errors in the echo cancellation signal.
In the adaptive transversal filter proposed by Agazzi et al, this problem is mitigated in that the transversal filter is constructed as a binary transversal filter, the binary signals which represent the send signal levels being directly applied to the delay means in the transversal filter and for each of the binary signals or signal combinations representing a send level, delay means are provided in the form of an N-bit shift register. Associated with each shift register is a coefficient register in which a coefficient is stored for each output signal of that shift register. After multiplication of all the output signals of the shift registers by the associated coefficients, all the multiplying signals are added together to obtain the echo cancellation signal. As each of the signal levels is cancelled thus via its own shift register and its own coefficient register, also signal level differences can be compensated for. The adaptive control loop then ensures an appropriate setting of the coefficients by which the different signal levels are to be multiplied to accomplish that the echo cancellation signal is such that the sum of the echo signal and the echo cancellation signal approximates the zero signal level to the best possible extent. A drawback of the prior art binary adaptive transversal filter is that, for example for three-level signalling, each of the coefficients in the coefficient register must in principle be able to assume a value which corresponds to the full amplitude of the send signal, which requires a great deal of storage capacity and consequently a large chip surface area when realized in integrated circuit techniques.